Doherty amplifiers are used on a relatively large scale in modern base stations that are used in mobile communications. A variant of Doherty amplifier may be adapted to be used in wideband operation in broadcast applications. The structure of such a wideband Doherty amplifier is shown in FIG. 1. The amplifier comprises a signal splitter 1 adapted to split an input signal into two almost equal amplitude components. A first component is inputted into a phase shifter 2 and the resultant phase shifted signal in further inputted into a Main amplifier 3. A second component is inputted into a Peak amplifier 7. Typically, the phase shift is 90°. In normal operation, the Main amplifier 3 operates in class AB and the Peak amplifier operates in class C. The two components of the input signal are amplified by the two amplifiers and at the output of the Main and Peak amplifiers respective amplified signals are obtained. The amplified signals are each phase shifted by phase shifters 4, 5 and 6 and the resultant signals are combined at point K. The phase shifters are also used as impedance adapters for load impedance 8 which is coupled to the point K.
Additionally, the Doherty amplifier may comprise adaptation circuits coupled to the phase shifters 4,5 and 6. The adaptation circuits may be, for example, LC filters.
The Main and the Peak amplifiers may each be implemented as power amplifiers typically as a single transistor implemented in different materials and technologies, such as for instance LDMOS, CMOS, GaN and GaAs.
Usually, the Doherty amplifier is mounted on a Printed Circuit Board (PCB) either single sided or double sided. In the case of amplifiers designed for relatively high frequency signals i.e. in the range of hundreds of MHz or more, the phase shifters 2, 4, 5, 6 may be implemented as delay lines; that is to say, they may be implemented as simple copper strips on one side of the PCB. The whole system may then be attached to a power dissipation radiator or heatsink, for dissipating the heat generated by the transistors, thereby protecting them from overheating.
The physical size of conventional wideband Doherty implementations is relatively large, which results in relatively high costs associated with the mechanical design of the transmitter. For amplifiers that are required to deliver relatively high powers i.e. in range of hundreds of watts, very large transmission lines are necessary.